The demand for increased light weighting in cars requires more sophisticated alloying concepts for high strength steels, by increasing mechanical resistance and by even lowering density. Alloying elements such as aluminum, manganese, silicon and chromium are first choice, but create severe problems in coatability caused by the presence of alloying elements oxides on the surface after annealing.
During heating the steel surface is exposed to an atmosphere which is non-oxidizing for iron but oxidizing for alloying elements with a high affinity towards oxygen such as manganese, aluminum, silicon, chromium, carbon or boron, which will provoke the formation of oxides of those elements at the surface. When the steel contains such oxidable elements, they tend to be selectively oxided at the surface of the steel, impairing wettability by the subsequent coating.
Moreover, when such coating is a hot dip coated steel sheet that is further heat treated for galvannealing, the presence of such oxides may impair the diffusion of iron in the coating which can not be sufficiently alloyed at the classical line speeds of an industrial line.